The present invention relates to a technique involved with a switching regulator operable to supply electric power, and particularly to a technique useful in application to a switching regulator controlled based on digital signal processing.
A power-supply circuit typified by a DC-DC converter need to be kept from supplying an output voltage thereby to reduce the power consumption when the power supply is not required. However, a power-supply circuit needs to rapidly raise and stabilize, at a predetermined level, its output voltage when they are required to supply electric power. Overshoot of the output voltage developed at the time of raising the output voltage would cause an excess voltage to be applied to a load device or a large amount of inrush current to pass therethrough when connecting the load device to an output terminal of the power-supply circuit, thereby breaking down the load device. On this account, it is necessary to rapidly raise an output voltage while avoiding its overshooting at the time of starting power supply.
In recent years, the system of controlling a negative feedback of a power-supply circuit has been in the process of change from analog to digital. For example, in an analog system, the difference between a set voltage and an output voltage is amplified and integrated, and the resultant signal is compared with a sawtooth signal by a comparator, whereby PWM (Pulse Width Modulation) signal is generated. Subsequently, the PWM signal thus generated is used to control ON/OFF of a power MOSFET of a switch circuit. The output voltage is varied by changing the ratio of ON time during which the power MOSFET stays in ON state to OFF time during which the power MOSFET is in OFF state, namely duty ratio, whereby the negative feedback is controlled so that the output voltage is coincident with the set voltage.
In contrast, in a digital system, operations e.g. amplification and integration are replaced with digital signal processing by DSP (Digital Signal Processor). Specifically, ADC (Analog to Digital Converter) converts an output voltage into a digital signal, and DSP processes the digital signal. Then, a digital PWM circuit, which generates a pulse having a width depending on the result of the signal processing, controls the duty ratio of the power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), whereby the negative feedback is controlled so that the output voltage is coincident with the set voltage. According to such digital system, a control parameter can be adjusted by a software program, and therefore a power-supply circuit can be flexibly designed according to its application and function. Further, a digital system can suppress the effects of manufacturing variations and environmental variations on the control characteristics in comparison to an analog system.
However, the stability margin of a negative feedback loop of a power-supply circuit according to a digital system is in danger of being deteriorated in comparison to that of an analog-based power-supply circuit. This is because a delay time which need not to be taken into account in the case of an analog system, e.g. time for conversion by ADC, and time for calculation by DSP, is figured in as factors of delay in a negative feedback loop in the case of a digital system. In other words, the deterioration of the stability margin of a negative feedback loop in a digital power-supply circuit can increase an overshoot and a ringing at the time of raising the output voltage, and in the worst case, it would cause the oscillation. Therefore, with a power-supply circuit for which the digital system is adopted, it is required to reduce an overshoot at the time of raising the output voltage even after the stability margin is deteriorated.
Conventional techniques for reducing an overshoot at the time of raising the output voltage include a technique referred to as “soft start”, by which the output voltage is raised forcibly and slowly. According to the “soft start” in the case of the analog system, the reduction in overshoot is generally achieved by raising the set voltage in a slope-like curve, e.g. charging an electric current into a capacitor thereby to produce a set voltage which varies in a slope-like curve. The non-patent document presented by L. Xinquan, G. Jianping, Y. Weixue, and C. Yu, “A novel digital soft-start circuit for DC-DC switching regulator”, International Conference on ASIC, pp. 564-567, 2005, discloses a technique for realizing the “soft start” by using a logic circuit and DAC (Digital to Analog Converter) to raise the output voltage stepwise, which eliminates the need for the capacitor.
Further, the Japanese Unexamined Patent Publication No. JP-A-2008-79483 discloses a technique for suppressing an overshoot of RPM (Revolutions Per Minute) of a motor. A motor driving circuit disclosed by JP-A-2008-79483 calculates a control amount depending on a digital value representing a quantity of error, which is the difference between the current RPM and target RPM, and controls the motor RPM to be the target value based on the calculated control amount, according to digital negative feedback control. The motor driving circuit sets the upper limit of the digital value at the time of calculating the control amount. As a result of so setting, the digital value never exceeds the upper limit no matter how large the value of the error is. Therefore the control amount is controlled appropriately, and thus the overshoot of the motor RPM can be suppressed.